When running Nagios 4 under SELinux on Centos, you might need to create some rules to get it to run. If running in permissive mode or with SELinux disabled, you can skip this.

Install the SELinux troubleshooting tools:

yum install setroubleshoot

Set SELinux to permissive mode so that Nagios will run:

setenforce 0

Now use Nagios for a while, trying all the different functions, this will make the SELinux system log issues that prevent
Nagios running under SELinux to your audit log file. After you have a good log file, run it through the auditing tools
and create modules to allow Nagios to run.